The excitation frequency, fe, of a flux switching permanent magnet (FSPM) machine is proportional to the number of rotor poles (pr), fe=npr/60, and not pole pairs, where n is the rotational speed in revolutions per minute (rpm). A typical FSPM machine has 12 stator slots and 10 rotor poles resulting in a high fundamental excitation frequency when operated at high-speed. For some high speeds, the fundamental frequency may not be attainable with today's power electronic converters.
To reduce the fundamental frequency for a given rotational speed, the number of rotor poles should be as small as possible. The minimum number of stator slots is six for a three-phase machine since it should be an even number and also a multiple of three. The number of rotor poles can be 4, 5, 7, 8, etc. Previously, not all of these combinations are suitable for practical use due to issues such as an unbalanced back-electromotive force (EMF) and unbalanced rotor force.